In an optical scanner used for an electrophotographic image forming apparatus, a light beam emitted from a light source is converted into a dotted image and focused on a target surface to be scanned, such as a peripheral surface of a photoconductor drum. The light beam is caused to rapidly sweep the peripheral surface of the photoconductor drum in an axial direction (main scanning direction) thereof. The optical scanner includes a light deflector for deflecting the light beam in the main scanning direction. The optical scanner further includes an incident optical system located upstream of the light deflector, and a scanning optical system located downstream of the light deflector. The incident optical system is configured to converge the light beam in the sub-scanning direction to bring the same to a focus in the proximity of the light deflector while rendering the light beam slightly convergent in the main scanning direction. Meanwhile, the scanning optical system focuses the light beam deflected by the light deflector on the target surface to form a dotted image.
To convert the light beam as described above, the incident optical system of the optical scanner uses an anamorphic condensing lens which has different powers (refractive powers) with respect to a main scanning direction and a sub-scanning direction. Further, in this anamorphic condensing lens, the wavefront aberration is corrected with respect to change in wavelength of a light beam on the order of 790 nm, at which a photoconductor drum shows good sensitivity. In this technique, since the magnitude of the wavelength change is small, it is not necessary to consider change in spherical aberration.
To evaluate transmission wavefront of an anamorphic condensing lens, an interferometer operating with a light source for emitting a light beam whose wavelength is on the order of 790 nm is conventionally used.